The present invention is directed to soot free autothermal reforming (ATR) of hydrocarbon feed.
In autothermal reforming, combustion of hydrocarbon feed is carried out with substoichiometric amounts of oxygen by flame reactions in a burner combustion zone and, subsequently, steam reforming of the partially combusted feedstock in a fixed bed of steam reforming catalyst. Substoichiometric combustion of hydrocarbons leads disadvantageously to formation of soot. Soot formation may be avoided by using a specific burner design and through controlling the operating conditions of the ATR process. Soot is formed in the flame of an autothermal reactor within certain ranges of operating conditions. When the amount of steam relative to the other components passed to the ATR reaction is under a certain critical value, soot is formed in the reacting feed. The limiting amount of steam can be expressed as the critical steam to carbon ratio, calculated as the molar flow rate of steam to the molar flow rate of carbon in the hydrocarbon feed. The hydrocarbon feedstock can be in form of natural gas or another kind of hydrocarbon including LPG, butane, naphtha, etc. The molar flow rate of carbon is calculated as the molar flow rate of the hydrocarbon times the carbon content of the hydrocarbon. The design of the burner nozzles has influence on the critical steam to carbon ratio. One such burner useful in ATR is described in U.S. Pat. No. 5,496,170.
Examples of operating conditions, which do not result in soot formation, are summarized in a paper by Christensen and Primdahl (Hydrocarbon processing, March 1994, pages 39-46). The conditions disclosed in this paper are shown in Table 1. Due to heat loss from the relatively small pilot unit employed in the experiments of Christensen and Primdahl to the adiabatic ATR exit temperature will be higher than the temperature given in Table 1. This means that if a large unit, from which the heat loss is negligible, is subjected to the exact same conditions, the ATR exit temperature will be close to the adiabatic ATR exit temperature.
Advantageously, the process is operated at low steam to carbon ratios, since a low ratio lowers the investment expenses for an ATR plant and reduces the necessary energy consumption in operating the plant. Additionally, a low steam to carbon ratio makes it possible to optimise the produced synthesis gas composition for production of CO-rich gases for e.g. methanol or dimethyl ether synthesis and Fisher-Tropsh processes.
The feed to the ATR is divided into two separate streams, which are send to the burner. One stream contains oxygen and steam. The other stream contains hydrocarbon and steam and optionally hydrogen and/or carbondioxide. The two streams are mixed downstream the burner and combustion takes place.
It has now been found that the critical steam to carbon ratio depends on the distribution of steam to the oxygen and hydrocarbon feed stream. When the amount of steam contained in the oxygen feed stream is reduced the critical steam to carbon ratio is reduced.
Based on the above findings, this invention provides a process for the preparation of a hydrogen and/or carbon-monoxide rich gas comprising the step of partial oxidation of a hydrocarbon feedstock with an oxygen containing reactant stream in presence of steam, wherein the steam is present in the hydrocarbon feedstock in an amount higher than in the oxygen containing reactant stream.
The invention allows operation of the process at a lower steam to carbon ratio compared to the critical steam/carbon ratio at equal distribution of steam between the oxygen and the hydrocarbon containing streams. The distance to the critical steam to carbon ratio is thereby by the invention reduced and the risk of soot formation in case of a change in operating conditions e.g. in case of an upset in the feed supply.
Below is the inventive process illustrated at a steam content in the oxygen containing stream down to an amount of 5%, but the process will be applicable also at lower steam concentrations.